1. Field of the Invention
The present invention relates to a spectral sensitivity adjusting mechanism for a light sensor.
2. Description of Related Art
A sensor of a measuring device for measuring light energy, such as a light sensor of an illuminance meter, a luminance meter for evaluating the luminosity factor, a densitometer, and a photoelectric colorimeter for evaluating color and a power meter for measuring an energy amount of a laser beam needs to be adjusted so as to provide a predetermined spectral sensitivity (which can be standard value, such as the ISO, the company standard and the product standard), for example, the spectral sensitivity as shown in FIG. 21. The spectral sensitivity of the light sensor is usually adjusted by correcting the synthetic spectral sensitivity to correspond with a predetermined sensitivity. The synthetic spectral sensitivity is generally created by a multiplication effect of the spectral sensitivities of several kinds of optical filters F1, F2, and F3 (such as a color glass filter, a film filter and an interference filter) and a photoelectric converting element SPD (such as a photoelectric cell, a photoelectric tube, a photomultiplier tube and a photodiode). The optical filters are inserted vertically in the optical path of the incident light before the photoelectric converting element SPD, as shown in FIG. 16. However, the optical filters F1 to F3 can frequently be unequal in their characteristics within manufacturing tolerances, as shown in FIG. 17 to FIG. 19. The photoelectric converting element can also be unequal in sensitivity between individual parts, as shown in FIG. 20. Therefore, even a light sensor formed of a combination of the same kind of optical filter and photoelectric converting element can be unequal in its spectral sensitivity, as shown in FIG. 5.
A color filter and a film filter can be adjusted in both their dispersion effect and their thickness during a manufacturing step of a light filter. As a result, any unequalness in the transmittance among the filters can be reduced to a certain degree. In a forming process of an interference layer on a substrate, a condition in forming a layer can be kept fixed, so that any unequalness in the transmittance among the filters can be reduced to a certain degree. However, in the present technology of filter manufacturing, when a light filter is used as a component of a light sensor without a selecting process, the light sensor cannot be set at a fixed predetermined spectral sensitivity. Therefore, in the present process of manufacturing a sensor having a predetermined spectral sensitivity, a manufacturer of a light sensor will buy only the optical filters capable of fulfilling the predetermined characteristics (transmittance characteristics), or after buying components, the manufacturer will carefully select the components for assembling so that the spectral sensitivity in the combination of an optical filter and a photoelectric converting element can fulfill an objective standard. For manufacturing products in which only a little unequalness among the sensors (unequalness among the devices) is desired, a highly precise optical filter is required to be used. Accordingly, a filter needs to be more strictly selected and the cost of components can become very high.
Generally, an optical filter is inserted vertically in the optical path of the incident light on the photoelectric converting element. But there are examples in which an interference filter can be inserted at an inclination to the optical path of the incident light on the photoelectric converting element. A cold mirror (which reflects longer wavelength light) or a hot mirror (which reflects shorter wavelength light) are such examples. An additional example is a half mirror which is an interference filter having wavelength selecting characteristics by inserting the interference filter at an inclination. Such examples are frequently utilized when the unequalness in the spectral transmittance characteristics (or the spectral reflectance characteristics) do not matter. Further, even if such a filter is inserted at an inclination in the optical path of the incident light on the photoelectric converting element, its spectral transmittance characteristics (or the spectral reflectance characteristics) is not generally adjusted by changing the inclination angle.